NTSC Composite Video Signals, and the RS - 170A StandardsPart 5 - The Horizontal Sync Pulse By pressing the " MAG " button ( for magnify ), we can zoom in on the Horizontal Sync pulse and Chroma Burst. Sometimes the button you are looking for may be called " MAG x 5 " or Horizontal Mag. Use common sense. After magnifying the signal, you may also need to adjust the horizontal or vertical positioning to view the picture as we see it above. Notice that the Chroma Burst now looks like a series of sine waves. That's because it is. It is a sine wave, with a frequency of 3.579545Mhz MHz. Sometimes this is shortened to 3.58 MHz, especially by folks in the consumer electronics business, but keep in mind that if the FCC asks, or if you are aligning a transmitter, 3.58 MHz is ILLEGAL! The correct Chroma Burst frequency is 3.579545Mhz MHz +/- 10 Hz. Not only is the Chroma Burst frequency important, but there is an exact phase and timing relationship which must exist between the Horizontal Sync Pulse and the Chroma Burst. Specifically, ( hang on to your hat ): "The zero crossings of reference subcarrier shall be nominally coincident witht the 50% point of the leading edges of all horizontal sync pulses. For those cases where the relationship between sync and subcarrier is critical for program integration, the tolerance on this coincidence is +/- 45 of reference subcarrier." 
Or more simply put: On line 10 of color field 1, the leading edge of the negative going sync pulse must cross the positive going chroma subcarrier at the 50% point.If we drop downward the chroma burst of a given signal, and extend it's length, the center of the the upward going leading edge of the chroma burst should cross the horizontal sync pulse at the center of the downward going leading edge of both signals. It is only when the leading edges of these two signals ( the chroma burst and the horizontal sync pulse ) cross at their center points, that we have a correct signal. And all signals being fed into a video switcher or router must cross at these points at the exact same time. Sounds difficult doesn't it? It isn't. But before we go into the adjustment of the signals to make them match, lets take another look at this zoomed in signal.  There are some terms that describe the parts of this signal that we should be familiar with. The specific relationships need not be memorized, but you should know where to find them for reference, should you need to.   BLANKING is the time period between video lines. When a horizontal line comes to the end, blanking comes on, and the dot on the screen is moved back from the right side of the screen to the left to begin drawing the next line. However, the dot is turned off, or blanked during this " reverse sweep " time, so it doesn't create an undesirable line on the screen. During this time, the signal rides at a zero IRE reference blacker than black level. This zero IRE reference level is called "blanking level". Black is actually 7.5 IRE above zero. The entire time period in which blanking is engaged is 10.9 micro-seconds ( μS ) and is broken down as in the chart above. Note that at least 3 parts of this signal relate to parts of a house. The Front Porch, of course is on the front of the blanking section. The Back Porch is, well, on the back. And finally, the breezeway, is, um.... in the middle. Now, for how to align two signals. |
This Course was written by Ray Dall © All Rights Reserved.
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